When you want to disinfect water with oxidants such as ozone, bromine, or chlorine, ORP can be used as a method to determine how effective the disinfection is. Different applications will require different ORP (mV) levels. This article will attempt to explain.
ORP stands for oxidation reduction potential. An ORP sensor (or probe) measures the ORP level in the water and displays a corresponding mV reading on the display of the ORP controller. The ORP reading provides a measure of the ability of one molecule to reduce or oxidize another molecule. In most applications, the ORP is a measure of the level of Oxidant, or Free Available Halogen, in the water. Applications for ORP control include cooling towers, swimming pools, water features and spas.
ORP is a potentiometric measurement in which the potential for electron transfer is sensed by an inert metal electrode (platinum or gold) and is read relative to an internal reference electrode - usually Ag/AgCl. The readout of the sensor is a voltage (relative to the reference electrode) with positive values resulting from an oxidising environment (ability to accept electrons) and negative values resulting from a reducing environment (ability to give electrons). ORP sensors have a similar construction to a pH sensor , except that instead of a glass bulb (which is specific for hydrogen ions as the basis for pH measurement) it has a platinum (or in some cases gold) electrode.
As pH is a direct, quantitative measurement of a dissolved species (hydrogen ion), the pH sensor is calibrated over a 2-point range i.e. pH 7.0 (essentially the "slope zero") and an alternative pH such as 4.0 or 10.0. The pH sensor will require routine recalibration as the concentration of it's electrolyte and the quality of the glass bulb decrease over time. ORP is not a quantitative measurement - it is merely a measure of a process liquid's capacity to accept or donate electrons. You are not actually measuring the concentration of any dissolved species - any millivolt readings obtained are merely qualitative measurements. Hence a single point "calibration" can only be performed with the assumption made that the ORP controller will read 0 mV at 0 mV. The millivolt potential measured by the controller is the difference of the ORP electrode and it's internal reference electrode, which typically has a potential of 200 mV at room temperature.
The primary problem with trying to calibrate an ORP controller in the field is that the range of ORP "calibration" solutions commonly available are not very accurate - with variations up to +/- 30 mV from the stated mV standard solution reading (e.g. 476 mV). When measuring ORP in your system, it is also important to control the pH level in order that the optimum levels of free halogen are available - especially for chlorine dosing applications.
The higher the pH, the lower the ORP set-point required to maintain the desired ppm of Free Chlorine. For instance, in a swimming pool, which is ideally controlled to 7.4pH, the ORP needs to be maintained at approximately 700mV to achieve the desired Free Chlorine level of 1 to 2 ppm. However, in a cooling tower, where the pH might hover around a pH of 8 - 8.5, you would lower the ORP set-point. Typical levels might be around 400mV to maintain an approximate 0.5 ppm of Free Chlorine. This relationship also varies due to other dynamics of the recirculating cooling water system, such as system volume, recirculation rate and temperature. In some cases you might have two cooling towers side by side at exactly the same pH. It is possible that you will require different ORP set-points to maintain the same free halogen levels in each system. This is a common, and acceptable, practice. It is recommended in all cases to ensure your controller is calibrated to a known mV source such as an electronic mV simulator. The use of ORP standard solutions should be limited to verifying that an ORP sensor is responding to changes in the overall ORP level.
Are you interested in pH Probes? We've just published a simple Guide to Understanding pH probes which you can download here
